1. Field of the Invention
The present invention relates to a compatible optical pickup using a light source emitting light of two different wavelengths and a method of aligning an optical axis in the compatible optical pickup.
2. Description of the Related Art
Optical recording and/or reproduction apparatuses for digital versatile disc family recording media (hereinafter, DVD) capable of performing high-density information recording and/or reproduction should generally also be compatible with compact disc family recording media (hereinafter, CD). the conventional CD has a standard thickness of 1.2 mm, whereas the DVD has a standard thickness of 0.6 mm in consideration of a disc tilt tolerance and a numerical aperture (NA) of an objective lens. Light sources for DVD and CD reproduction have different wavelengths. Reproduction of the CD uses light having a wavelength of about 780 nm, and reproduction of the DVD uses light having a wavelength of about 650 nm. With regard to a CD-R having an organic dye layer as a recording layer, its data are damaged when using light of a 650-nm wavelength. therefore, an optical recording and/or reproduction apparatus for a DVD usually uses a compatible optical pickup using light of two different wavelengths to perform recording and/or reproduction on the CD family recording media, including the CD-R.
As shown in FIG. 1, a compatible optical pickup using two light sources 1 and 3, which emit different wavelengths of light, and one photodetector 19 has been suggested in the prior art. Referring to FIG. 1, the conventional compatible optical pickup includes a first light source 1 for emitting light of a 650-nm wavelength and a second light source 3 for emitting light of a 780-nm wavelength. Light emitted from the first light source 1 is used to record information on and/or reproduce information from a DVD 10a having a relatively small thickness. Light emitted from the second light source 3 is used to record information on and/or reproduce information from a CD 10b having a relatively large thickness.
The light emitted from the first light source 1 is reflected by a plate-type beam splitter 7, is transmitted through a cubic beam splitter 9, and proceeds toward a disc 10, which is a received one of the CD 10b and DVD 10a. the light emitted from the second light source 3 is reflected by the cubic beam splitter 9 and proceeds toward the disc 10. An objective lens 15 focuses light incident from the first and second light sources 1 and 3 to form a light spot on the disc 10. the light emitted from the first light source 1 is focused on the DVD 10a having a relatively small thickness, and the light emitted from the second light source 3 is focused on the CD 10b having a relatively large thickness. Light reflected from the disc 10 is sequentially transmitted through the objective lens 15, the cubic beam splitter 9, and the plate-type beam splitter 7, and is received by a photodetector 19.
In FIG. 1, a grating 5 is used to diffract the light from the second light source 3 to be 0th-, +1st-, and −1st order diffracted beams so as to detect a tracking error signal by a three-beam method during recording and/or reproduction. A collimating lens 11 collimates diverging light from the first and second light sources 1 and 3. A light sensing lens 17 condenses the light incident through the cubic beam splitter 9 after being reflected by the disc 10 such that the light is received by the photodetector 19.
Since the conventional optical pickup having the above structure includes the two light sources 1 and 3, it is compatible with both the DVD 10a and the CD 10b. However, the conventional compatible optical pickup uses the two light sources 1 and 3 which are separately disposed, which complicates the optical axis alignment with respect to the first and second light sources 1 and 3 and an assembly process.
In particular, in the compatible optical pickup using the two light sources 1 and 3, after the optical elements used with the DVD 10a are adjusted for an optimal DVD optical axis alignment, the optical elements used with the CD 10b are adjusted for a CD optical axis alignment. The optical axis of the first light source 1 for the DVD 10a is first adjusted, and the optical axis of the second light source 3 for the CD 10b is adjusted. Since the first and second light sources 1 and 3 are located at different positions, an adjusting point is different for the first and second light sources 1 and 3. After the DVD optical axis alignment, the second light source 3 is adjusted in three dimensions, which is difficult to control for the CD optical axis alignment the three-dimensional adjustment of the second light source 3 for the CD optical axis alignment is needed because the optical axis for the CD is affected by multiple parameters, such as the position and angle of the second light source 3 and the position and angle of the cubic beam splitter 9. As a result, the assembly and optical axis alignment processes for the conventional compatible optical pickup including the two light sources 1 and 3 are complicated. In addition, a tilt of the objective lens 15 finally adjusted to be aligned with the DVD optical axis does not match the optical axis of the second light source 3 for the CD. the conventional compatible optical pickup using two light sources, having the above drawbacks, reduces process reproducibility and productivity and increases the manufacturing cost.
The arrows shown in FIG. 1 indicate the directions in which the optical devices are adjusted for the CD optical axis alignment. In particular, to align the optical axis for the CD, the second light source 3 is adjusted in three dimensions, the grating 5, the objective lens 15, and the photodetector 19 are adjusted in two dimensions, and the cubic beam splitter 9 and the light sensing lens 17 are adjusted in one dimension.
FIG. 2 shows another example of a conventional compatible optical pickup including a light source 20 for emitting first and second lights I′ and II′ having different wavelengths. The lights I′ and II″ are emitted separated a predetermined interval from one another. A hologram coupler 25 compensates for the distance between the optical axes of the first and second lights I′ and II″ having different wavelengths and is currently being developed. In FIG. 2, the same reference numerals as those in FIG. 1 denote the same or similar functional elements as those in FIG. 1, and thus descriptions thereof are not repeated here.
The light source 20 is a 2-wavelength-1-laser light source and emits the first light I′ of a wavelength of about 650 nm and the second light II′ of a wavelength of about 780 nm. The first light I′ is used for recording and/or reproduction on the DVD 10a, and the second light II′ is used for recording and/or reproduction on the CD 10b. When the hologram coupler 25 is disposed between the light source 20 and a plate-type beam splitter 27, the light source 20 is constructed to emit the second light II′ at a predetermined angle with respect to the first light I′. the hologram coupler 25 transmits the first light I′ incident perpendicular to one side and diffracts and transmits most of the second light II′ incident thereon at an angle such that the first light I′ and the second light II′ proceed parallel to each other.
Alternatively, the light source 20 can be constructed to emit the first and second lights I′ and II′ parallel to each other with a predetermined separation from each other. In this case, the hologram coupler 25 is disposed between the plate-type beam splitter 27 and the photodetector 19.
When the hologram coupler 25 is disposed between the plate-type beam splitter 27 and the photodetector 19, as shown in FIG. 3, the hologram coupler 25 transmits the first light I′ between the first and second lights I′ and II′, which are incident thereon parallel to each other with the predetermined separation from each other, and diffracts and transmits the second light II′. As such, the first light I′ and the second light II′ are received together at the same portion of the photodetector 19.
In FIGS. 2 and 3, the arrows indicate the directions in which the optical elements are adjusted to align the optical axis for the DVD and the CD. In the conventional compatible optical pickups described with reference to FIGS. 2 and 3, the first and second lights I′ and II′ that proceed separated a predetermined interval from one another are adjusted to be aligned with the same axis using the hologram coupler 27 so as to proceed parallel to each other or to be gathered on the photodetector 19. In these structures, the optical axis for the DVD that has been aligned is changed as a result of the optical axis alignment for the CD. therefore, the optical axis alignment for the DVD and CD needs to be repeatedly performed.